Mozilla developers have decided to switch from doing frequent planned releases of Firefox to a variable schedule. From now, Firefox will quit the version race started by Chrome and will be released less frequently.
Previously, a new version of the Firefox browser was released every 6 weeks. From now, new Firefox versions will be developed within a six-to-eight week release cycle. According to Mozilla, an additional two weeks will be dedicated to every release with an odd version number. This will allow them to perform more rigorous testing and fix unexpected bugs in the browser.
So, every odd numbered release of Firefox will get a full 6 weeks of development and 2 weeks for testing. This should increase the quality of the browser code base, since the beta and nightly branches will also benefit from the additional development and testing time.
At the end of the year, there will be a minor release with critical bug fixes only instead of a complete release. This will allow Mozilla developers to take a little break during the New Year holidays.
The previous release cycle will be followed till Firefox 46. So it will be as follows:
2016-03-08 – Firefox 45, ESR 45 (6 weeks)
2016-04-19 – Firefox 46 (6 weeks)
2016-06-07 – Firefox 47 (7 weeks)
2016-08-02 – Firefox 48 (8 weeks)
2016-09-13 – Firefox 49 (6 weeks)
2016-11-08 – Firefox 50 (8 weeks)
2016-12-13 – Firefox 50.0.1 (5 weeks - release for critical fixes as needed)
2017-01-24 – Firefox 51 (6 weeks from prior release)
You can find the actual release calendar on this wiki.